Population Genetics

Mendelain populations and the gene pool

Inheritance and maintenance of alleles and genes within a population of randomly breeding individuals

Study of how often or frequent genes and/or alleles appear in the population

Genotypic frequencies – how often do certain allelic combinations appear

Allelic frequencies - how often does an individual allele appear

Genotypic frequenciesfrequency a particular genotype appears (combination of alleles)for moths at rightout of 497 moths collectedBB appears 452 timesBb appears 43 timesbb appears 2 times

FrequenciesBB 452 ÷ 492 = 0.909Bb 43 ÷ 492 = 0.087bb 2 ÷ 492 = 0.004Total 1.000

BB

Bb

Bb

bb

What about alleles that do show simple dominant - recessive relationship?

How does genotypic frequency really demonstrate flux or change in frequencies of the dominant allele?

What if there are multiple alleles?

Allelic frequencies

Allelic frequency

Allelic frequency = Number of copies of a given allele divided by sum of counts of all alleles

BB appears 452 timesBb appears 43 timesbb appears 2 times492 moths 994 allelesFrequenciesB (904 + 43) ÷ 994 = 0.953b (43 + 4) ÷ 994 = 0.047Total 1.000

BB

Bb

Bb

bb

Can also calculate it from the genotypic frequencies

BB was 0.909

Bb was 0.087

bb was 0.004

Therefore frequency of B = Frequency of BB + ½ frequency of Bb

f(B) = .909 + ½ 0.087 = .909 + .0435 = .9525

F(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047

What about multiple alleles?

Genotype Number

A1A1 4

A1A2 41

A2A2 84

A1A3 25

A2A3 88

A3A3 32

Total 274

f(A1) = Total number of A1 in population divided by total number of alleles

Genotype Number

A1A1 4

A1A2 41

A2A2 84

A1A3 25

A2A3 88

A3A3 32

Total 274

f(A1) = Total number of A1 in population divided by total number of alleles

Genotype Number Number of A1

A1A1 4 2 X 4

A1A2 41 41

A2A2 84

A1A3 25 25

A2A3 88

A3A3 32

Total 274

f(A1) = ((2 X 4) + 41 + 25) ÷ (2 X 274)

= (8 +41 + 25) ÷ 548

= 74 ÷ 548

= 0.135

Allelic frequencies at X linked locussame principle

However remember for humans that males only have one X

So that

F(one allele = 2 X the homzygous genotype) + the number of heterozygotes + the males with the phenotype all divided by the number of alleles in the population (2 X females) plus males.

Hardy – Weinberg “law”

Frequencies of alleles and genotypes within a population will remain in a particular balance or equilibrium that is described by the equation

Consider a monohybrid cross, Aa X Aa

Frequency of A in population will be defined as p

Frequency of a in population will be defined as q

Gametes A (p) a (q)

A (p) AA(pp) Aa(pq)

a (q) Aa(pq) aa(qq)

Frequency of AA offspring is then p2

Frequency of aa offspring is then q2

Frequency of Aa offspring then 2pq

Frequency of an allele being present is = 1

p2 + 2pq + q2 = 1

Where p = frequency of “dominant” allele

and q = frequency of “recessive” allele

For the moth example

(0.9525)2 + (2 X (0.953 X 0.047)) + (0.047)2

0.907 + (2 x 0.045) + .002

.907 + .09 + .002 = .999

Is this good enough?

Can be extended to more than two alleles

Two alleles

(p + q)2 = 1

Three alleles

(p + q + r)2 = 1

And X – linked alleles

Can be used to det4ermine frequencies of one allele if the presence of one allele is known

Conditions or assumptions for the Hardy – Weinberg law to be true

Infinitely large population (?)

Randomly mating population (with respect to trait)

No mutation (with respect to locus or trait)

No migration (with respect to locus or trait)

No natural selection (with respect to locus or trait)

Frequencies of alleles do not change over time

Population variation

How is it quantitated?

Proportion of polymorphic loci

Heterozygosity

Population variation in space and time for allelesBlue mussel

Cline –systematic variation in allele frequency across geography

Temporal variation

Population variation

Variation at many loci

How is it detected?

PCR

Sequencing

Protein electrophoresis

VNTRs

SNTRs

Synonymous vs. non-synonymous variations or chnages

How is population variation of loci obtained

Random events

Mutation

Gain and loss of genes from the gene pool

Founder effect

Bottleneck effect

Random genetic drift

Selection

Migration

Mutations may be lost or fixed within a population

Selection and speciation

Selection coefficient

Heterozygote superiority

Selection against recessive lethal

Fitness

Problems

Text Study Guide

22.1 - 22.5 pg 502 – 505

1-15

Terms

Mendelian population

Gene pool

Genotypic frequencies

Hardy-Weinberg law

Genetic drift

Random mating

Cline

Random genetic drift